In some active microwave systems it is necessary to use limiter circuits having the function of delivering constant output power regardless of the power applied to the input. Such circuits are nowadays generally constituted by field effect transistors (FETs) used under saturation conditions beyond their compression points.
In addition, in certain specific applications it is appropriate for the phase of the signal delivered by the limiter circuit to remain constant in spite of variation in the input signal. However, experience shows that sometimes the use of certain types of field effect transistors makes it possible to satisfy this requirement whereas other types of field effect transistors are not suitable for overcoming phase variations that may be as much as several degrees.
The initial idea consists in making a limiter circuit using a field effect transistor that provides good performance with respect to variation in its insertion phase: i.e. the ratio of input signal phase to output signal phase of the limiter circuit as a function of the power injected to its input.
The major portion of the variation in insertion phase observed on limiter circuits using field effect transistors is due to the phase shift introduced by such transistors when they are used in their compression zone, i.e. in the zone for which their output power no longer varies for increasing input power.
An object of the invention is thus to provide a limiter circuit using a field effect transistor and enabling the insertion phase variation introduced by field effect transistors operating under saturated conditions to be minimized.
There is very little published work, at present, on this subject.
Only one article, by Serge Bertrand, entitled (in translation) "An FET amplifier-limiter without phase shift--application to the 7.9 GHz to 8.4 GHz band", and published at pages 146 and 147 of (in translation) "The National Microwave Symposium" held at Nice, France June 1987, which article describes the phase variations of a field effect transistor as a function of input power and of the impedance present at its output, demonstrates the feasibility of a limiter circuit based on a field effect transistor and having optimized phase shifting. The article also describes a module suitable for being cascaded and a limiter circuit having a phase shift of less than 2.degree. over a dynamic range of 21 dB in the 8.1 GHz to 8.3 GHz band.
That article thus describes making a limiter circuit using an AsGa field effect transistor having excellent performance with respect to insertion phase variation (less than 2.degree. for input power varying from Pe=-.infin. to Pe=Pe.sub.1dB +10 dBm). In order to achieve this result, that article does not describe a circuit technique per se, but a method of prior sorting transistors to select those transistors which have minimum insertion phase variation. Transistors selected in this way are then used to make the limiter circuit.
In contrast, the invention makes it possible, a priori, to limit insertion phase variation of the field effect transistors used to perform the limiting function, thereby making it possible to avoid this prior sorting technique.